This invention relates to Broadband (e.g., zero to 100 KHz) transmission lines and shieldings therefor. More specifically, it elates to an improved type of shielded cable capable of continuous dependable service at extreme temperatures or under conditions of fluctuating high electrical and magnetic fields, yet quite flexible, with a high degree of flexing endurance and compact construction. This invention also relates to a communication system allowing the exchange of information between a central station and several data acquisition and transmission stations via a signal transmission line or cable in the borehole logging field.
In that borehole logging field, logging measurements of the characteristics of the different earth formations traversed by a borehole are generally carried out by lowering into the borehole a tool suspended by a cable. The cable is provided with a single or several electric wires for the transfer of data signals from the tool to a reception station on the surface. In order to reduce logging time, it is necessary to perform the measurements as rapidly as possible. To achieve this, it is customary to simultaneously lower into the borehole several tools. The information from the different tools is sampled and transmitted to the surface by means of a communication system. There are known communication systems specially designed to carry to the surface the logging data sent by tools lowered into boreholes. One such system is described, for example, in the U.S. Pat. No. 4,355,310 issued Oct. 19, 1982 to Belaigues et al and commonly assigned along with this application to Schlumberger Technology Corporation. The disclosure of the aforementioned U.S. Pat. No. 4,355,310 is incorporated herein by reference and made a part hereof.
Whenever several tools are lowered at a time into a borehole, these tools are connected end to end if this is possible. The tools are coupled over a bi-directional bus which extends from one end of the downhole apparatus to the other substantially as described in the Belaigues et al patent. Preferably, each tool is provided with a "universal" (i.e., identical) interface which is connected to the bi-directional bus in parallel. The bidirectional bus provides the sole transmission path for the information-bearing and control signals transmitted between the tools and a downhole controller coupled to the cable wires. The term "data signals" is used in this sense to mean signals which carry instructions, data, addresses and the like. In other words, all the connections to the tools, with the exception of power connections, are made through the bus.
This coupling of the tools via the bus creates some difficulty as the data signals being carried by the bus are affected by high magnetic and electric fields, e.g., due to power supplies, etc., which must be shielded against. Further, since space in the downhole tools is at a premium, the design of the shielding must be such as to economize space and allow for a high degree of flexibility and durability of the cable while preserving the integrity of the shield. These problems are exacerbated as power availability downhole is limited with the result that the data being carried along the bus lines is usually characterized by a low signal to noise ratio.
Various cable shielding arrangements have been attempted by the prior art. Notable among which is a silver coated copper wire material which is formed in a cable braid by contiguous helical windings around the cable to be shielded. Another type of material used specifically for magnetic shielding is amorphous material with soft magnetic behavior also generally known under the name "Metglas" which is a Trademark of Allied Signal Corporation. These Metglas materials are described along with their uses in an article by Ball et al, entitled "Application of Amorphous Magnetic Materials in Electronics," published in IEEE Transactions on Magnetics 6 (1981).